1. Technical Field
The present invention relates to a counter control circuit for digital counters and, more particularly, relates to a counter control circuit for controlling counting operation of a counter to accurately measure a pulse period of a video synchronization signal in a video display monitor.
2. Related Art
Conventional digital systems use a synchronization signal to control the timing of the execution of a command and to perform a desired operation during a pulse duration. The synchronization signal is formed by a series of logical high or low level pulses and can be periodically generated for enabling selective performance of a series of specific operations. Thus, each mode of operation can be changed by varying a pulse duration of each pulse of the synchronization. A typical request for changing an operation mode by varying the pulse duration can be exemplified by a horizontal synchronization signal and a vertical synchronization signal used in a video display monitor. That is, for example, an application program contained in a computer system such as a PC provides different image data by changing the operation mode of a video synchronization signal.
Meanwhile, modern video display monitors require many functional operations adapted to multimedia environment. One solution to multi-functional operations is to enable the display monitors to perform each operation based on an input mode of the video synchronization signal fed from the computer system. A video synchronization signal is discriminated such that if an excessive level of the video synchronization signal is entered into the display monitor, the input of the video synchronization signal is cut off to prevent the display monitor from being damaged. Thus, in order to discriminate the mode of operation based on the video synchronization signal, each pulse period of the video synchronization signal must be measured. Contemporary pulse measurement techniques are disclosed, for example, in U.S. Pat. No. 4,722,004 for Video Signal Discriminating Apparatus issued to Miyamoto et al., U.S. Pat. No. 5,134,481 for Method Of Automatically Measuring The Horizontal Scan Frequency Of A Composite Synchronism Signal, And A Electronic Circuit Operating In Accordance With The Method issued to Gornati et al., U.S. Pat. No. 5,175,620 for Synchronism Detecting Circuit Utilizing Pulse Width issued to Kim.
Many contemporary pulse measurement techniques, however, contain a number of drawbacks. First, the count value for pulse measurement is typically not accurate. Second, the starting point of a counting operation is generally indefinite.